[PATCH] jbd2: Fix use-after-free of transaction_t race

[Ritesh Harjani <riteshh@linux.ibm.com>]

jbd2_journal_wait_updates() is called with j_state_lock held. But if
there is a commit in progress, then this transaction might get committed
and freed via jbd2_journal_commit_transaction() ->
jbd2_journal_free_transaction(), when we release j_state_lock.
So check for journal->j_running_transaction everytime we release and
acquire j_state_lock to avoid use-after-free issue.

Fixes: 4f98186848707f53 ("jbd2: refactor wait logic for transaction updates into a common function")
Reported-and-tested-by: syzbot+afa2ca5171d93e44b348@syzkaller.appspotmail.com
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
---
 fs/jbd2/transaction.c | 41 +++++++++++++++++++++++++----------------
 1 file changed, 25 insertions(+), 16 deletions(-)

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 8e2f8275a253..259e00046a8b 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -842,27 +842,38 @@ EXPORT_SYMBOL(jbd2_journal_restart);
  */
 void jbd2_journal_wait_updates(journal_t *journal)
 {
-	transaction_t *commit_transaction = journal->j_running_transaction;
+	DEFINE_WAIT(wait);

-	if (!commit_transaction)
-		return;
+	while (1) {
+		/*
+		 * Note that the running transaction can get freed under us if
+		 * this transaction is getting committed in
+		 * jbd2_journal_commit_transaction() ->
+		 * jbd2_journal_free_transaction(). This can only happen when we
+		 * release j_state_lock -> schedule() -> acquire j_state_lock.
+		 * Hence we should everytime retrieve new j_running_transaction
+		 * value (after j_state_lock release acquire cycle), else it may
+		 * lead to use-after-free of old freed transaction.
+		 */
+		transaction_t *transaction = journal->j_running_transaction;

-	spin_lock(&commit_transaction->t_handle_lock);
-	while (atomic_read(&commit_transaction->t_updates)) {
-		DEFINE_WAIT(wait);
+		if (!transaction)
+			break;

+		spin_lock(&transaction->t_handle_lock);
 		prepare_to_wait(&journal->j_wait_updates, &wait,
-					TASK_UNINTERRUPTIBLE);
-		if (atomic_read(&commit_transaction->t_updates)) {
-			spin_unlock(&commit_transaction->t_handle_lock);
-			write_unlock(&journal->j_state_lock);
-			schedule();
-			write_lock(&journal->j_state_lock);
-			spin_lock(&commit_transaction->t_handle_lock);
+				TASK_UNINTERRUPTIBLE);
+		if (!atomic_read(&transaction->t_updates)) {
+			spin_unlock(&transaction->t_handle_lock);
+			finish_wait(&journal->j_wait_updates, &wait);
+			break;
 		}
+		spin_unlock(&transaction->t_handle_lock);
+		write_unlock(&journal->j_state_lock);
+		schedule();
 		finish_wait(&journal->j_wait_updates, &wait);
+		write_lock(&journal->j_state_lock);
 	}
-	spin_unlock(&commit_transaction->t_handle_lock);
 }

 /**
@@ -877,8 +888,6 @@ void jbd2_journal_wait_updates(journal_t *journal)
  */
 void jbd2_journal_lock_updates(journal_t *journal)
 {
-	DEFINE_WAIT(wait);
-
 	jbd2_might_wait_for_commit(journal);

 	write_lock(&journal->j_state_lock);
--
2.31.1